Potassium hexatitanate whiskers having a tunnel structure

ABSTRACT

A potassium hexatitanate whisker having a tunnel structure containing aluminum and niobium impurities in such amounts that Al 2  O 3  /Nb 2  O 5  molar ratio is 0.6 or higher and composite materials using the same. Also disclosed are processes for producing composite materials using the potassium hexatitanate whiskers having a tunnel structure with a light alloy or a thermoplastic resin.

BACKGROUND OF THE INVENTION Industrial Field of the Invention

The present invention relates to a novel type of whiskers and a fiberreinforced composite material using the same. More particularly, thepresent invention relates to potassium hexatitanate whiskers having atunnel structure and containing aluminum and niobium, both of which areimpurities, in such amounts that the Al₂ O₃ /Nb₂ O₅ molar ratio is atleast 0.6. The present invention also relates to a composite materialcomprising said whiskers as a reinforce material and a thermoplasticresin or a light alloy as a matrix.

Whiskers such as silicon carbide whiskers, silicon nitride whiskers,aluminum borate whiskers, and potassium titanate whiskers are knownheretofore. However, only potassium titanate whiskers among them are putto practical use as a reinforcing material for general use plastics aswell as for engineering plastics. This is ascribed to the fact thatwhiskers other than potassium titanate whiskers are too expensive. Theleast expensive potassium titanate whiskers, however, are stillexpensive as compared with glass fibers and wollastonite which arewidely used as reinforcing materials for plastics. The application ofthe composite materials containing potassium titanate whiskers atpresent is therefore confined to an extremely narrow field. Accordingly,from the viewpoint of extending the use of the known composite materialsand of acquiring the role of a reinforcing material for plastics, it isdesirable for production cost of potassium titanate whiskers to besubstantially lowered to a price comparable to that of glass fibers, orto develop potassium titanate whiskers having a higher strength withoutincreasing production costs.

With respect to the aim of providing low price potassium titanatewhisker reinforced plastics, some developments thereon have been made asdescribed, for example, in Japanese Patent Public Disclosure (Kokai)Nos. 60-51615 and 60-191019. However, there has been made no attempt toprovide advanced composite materials having a higher mechanical strengthat a low price, by taking advantage of whiskers having an improvedstrength.

Furthermore, potassium titanate whiskers are now attracting attention asa reinforcing material for light alloys to provide a general useindustrial material. This tendency is ascribed to the fact that thereinforcing materials suited for reinforcing alloys, i.e., the ceramicfibers, are quite expensive as compared with potassium titanatewhiskers. Hence, the use of a relatively cheap potassium titanatewhiskers would provide a low-price composite material having a widerange of application. With respect to the production process,squeeze-casting method is most favorable from the viewpoint of theperformance of the resulting composite material and of production costs.This process is advantageous in that the reaction can be accomplished ina short period of time and carried out with relatively simplemanufacturing apparatus. Thus, reinforced composite materials whichsuffer less problems concerning reaction of the reinforcing material andthe light alloy matrix can be obtained by a relatively simple process.

It was reported that potassium titanate whiskers react vigorously withan molten aluminum (see Hideharu Fukunaga, Michifumi Takeichi, andNobuhiro Ohta, J. Jpn. Soc. Compos. mater, 8(1982), p.66), but thisproblem was overcome by an improved potassium titanate whisker disclosedin Japanese Patent Public Disclosure (Kokai) No. 1-301516, as potassiumhexatitanate whiskers having a tunnel structure and a free potassiumcontent of 5 ppm or less.

The potassium titanate whiskers above were obtained by considerablyreducing the content of layered structure potassium titanate included inthe potassium hexatitanate whiskers having a tunnel structure. Thedevelopment of such potassium titanate whiskers has enable their use asa reinforcing material for light alloys with a view to providing generaluse industrial materials (see K. Suganuma, T. Fujita, K. Niihara, T.Okamoto, and S. Suzuki, J. Mater. Sci. Letters, 8 (1989) p.808).

The metal-based reinforced material using the above potassium titanatewhiskers as the reinforcing material exhibited improved mechanicalstrength as compared with the light alloy matrix composite using theconventional potassium titanate whiskers, however, it still wassubjected to the disadvantages inherent to the composite using theconventional potassium titanate whiskers. For example, when a heattreatment was applied to the composite material with an aim toimproving, for example, mechanical strength, resistance against stresscorrosion cracking, and the like, the potassium titanate whiskersdecreased strength and/or exhibited no improvement In the intendedproperties. Further, Suganuma et al. found magnesium-based very narrowreaction layer below 10 nm, was formed on the surface of potassiumtitanate whiskers during a production of a composition of potassiumtitanate whiskers and an aluminum alloy. They pointed out that thestrength of the matrix is thereby reduced, because magnesium content inthe matrix alloy is reduced. However, they also reported that thisreaction layer remained unchanged through the heat treatment processafter producing the composite, and hence it was still unclear why thestrength of the potassium titanate whisker reinforced light alloy shouldbe reduced by the heat treatment.

As described above, the conventional potassium titanate whiskerreinforced metal composites were not favorable for use in an industrialmaterial due to their lack of stability during a heat treatment, andhence the development of a novel and economically advantageous compositematerial capable of increasing its strength by a heat treatment, i.e., acomposite material having thermal stability, was not accomplished.

An object of the present invention is to provide more economically acomposite material having improved mechanical strength as compared withconventional types, by improving the mechanical strength of potassiumtitanate whiskers.

SUMMARY OF THE INVENTION

In the light of the circumstances described above, the present inventorshave conducted an extensive study to overcome the problems. As a result,it has been found effective to use, as a reinforcing material, potassiumhexatitanate whiskers having a tunnel structure and containing aluminumand niobium, both of which being impurities, in such amounts that theAl₂ O₃ /Nb₂ O₅ molar ratio is at least 0.6. The present invention hasbeen accomplished based on these findings.

More specifically, the present invention has been accomplished based onthe following findings: that conventional potassium titanate whiskersinevitably include impurities such as niobium, phosphorus, aluminum,silicon, iron, and alkaline earth metals, whose origin can be found inthe starting materials, and that such impurities cause a reduction instrength of the composite materials during heat treatment; and further,that such a loss of strength during the heat treatment of a compositematerial containing impurities can be avoided by controlling the amountof niobium and aluminum in potassium titanate whiskers as such to givethe Al₂ O₃ /Nb₂ O₅ molar ratio 0.6 or higher. The conventional potassiumtitanate fibers as a reinforcement are synthesized from startingmaterials that have high contents of impurities such as niobium,aluminum, phosphorus, calcium, magnesium, iron and silica, asexemplified by titanium oxide in anatase form produced by the sulfateprocess, metatitanic acid, and naturally occurring rutile sand oranatase sand. Potassium titanate fibers synthesized from these startingmaterials contain large amounts of impurities. Thus, the resultingpotassium titanate whiskers obtained therefrom inevitably contain aconsiderable amount of impurities, and hence a high strength cannot beachieved in such products. Furthermore, aluminum alloys reinforced withsuch potassium titanate whiskers having a high impurity content suffer alarge reduction in strength during the heat treatment thereof. However,it has been also found that potassium titanate whiskers containingniobium and aluminum at a controlled Al₂ O₃ /Nb₂ O₅ molar ratio of 0.6or higher yield an increased strength, and that plastics having beenreinforced with the same can also exhibit high strength. Furthermore, ithas been found that such whiskers, when used as a reinforcing materialof an aluminum alloy, avoid a reduction in strength of the compositematerials containing a large amount of impurities during the heattreatment thereof. The present invention has been accomplished based onthe basis of the findings stated above.

The present invention provides a composite material comprising athermoplastic resin matrix or a light alloy matrix, reinforced withpotassium hexatitanate whiskers having a tunnel structure and containingaluminum and niobium impurities in such amounts that the Al₂ O₃ /Nb₂ O₅molar ratio is at least 0.6.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically, the shape of a test piece to be subjected toa tensile test.

DETAILED DESCRIPTION OF THE INVENTION

The potassium titanate whiskers according to the present invention havea tunnel structure and containing aluminum and niobium, both of whichare impurities in such amounts that the Al₂ O₃ /Nb₂ O₅ molar ratio is atleast 0.6. Most preferably, they are potassium hexatitanate whiskershaving a tunnel structure and containing aluminum and niobium, both ofwhich are impurities in such amounts that the Al₂ O₃ /Nb₂ O₅ molar ratiois at least 0.6 , with the total content of impurities other thanaluminum and niobium being controlled to 1% or less in terms of oxides,and furthermore, they are single crystal whiskers free from other phasessuch as layered structure potassium titanate and titanium dioxide in theinside or on the surface of said whiskers. If layered structurepotassium titanate or titanium dioxide were to be incorporated in thewhiskers, the whiskers themselves become reduced in strength as comparedwith the comparative single crystal whiskers. Moreover, because suchlayered structure potassium titanate and titanium dioxide have extremelyhigh reactivity with light alloys as compared with potassiumhexatitanate whiskers having a tunnel structure, the whiskers wouldsuffer considerable damage at making the composite with the light alloymatrix or at the subsequent heat treatment process. Thus, such potassiumtitanate whiskers are unsatisfactory.

The potassium titanate whiskers according to the present invention canbe produced representatively by the following process:

First, a titanium feed compound and a potassium feed compound areblended in such proportions as to provide a composition represented bythe general formula K₂ O.nTiO₂ (n=2-4); the resulting mixture is bakedat 900°-1100° C. to yield masses of potassium titanate whiskers; themasses of potassium titanate whiskers are dipped in cold or warm waterso that they are disintegrated into individual single whiskers;thereafter, an acid is added to the slurry of individual single whiskersand its pit is adjusted to between 9.3 and 9.7, whereby the compositionof potassium titanate whiskers is transformed to a TiO₂ /K₂ O molarratio of 5.9-6.0; subsequently, the whiskers are heated at 900°-1150° C.For at least 1 h; finally, the whiskers are washed with an acid.

Examples of the titanium feed compound that can be used to produce thepotassium titanate whiskers include hydrated titanium oxides, titaniumdioxide in anatase form, titanium dioxide in rutile form, and rutileores. The potassium feed compound slay be selected from among thosecompounds which generate K₂ O during sintering, as exemplified by K₂ O,KOH, K₂ CO₃ and KNO₃. While niobium impurity in the starting material isalmost wholly included in the crystal lattice of the potassium titanatewhiskers to form a solid solution therewith, aluminum is only partiallyincorporated in the crystal lattice to make solid solution, because apart of the incorporated aluminum forms a compound with potassium.Accordingly, aluminum and niobium contents in the starting material mustbe controlled in such amounts that the Al₂ O₃ /Nb₂ O₅ molar ratio is atleast 0.7. The aluminum which does not constitute the crystal structureof the potassium titanate whisker and reacts with a potassium has noeffect on improving the properties of the whisker, furthermore thereacted aluminum compound reacts with potassium titanate whiskers anddamages them during the conversion step of the whiskers from layeredstructure to the tunnel structure. Thus, it is preferred that suchaluminum is separated and removed. This can be done in the step ofimmersing the synthesized masses of the potassium titanate whiskers intocold or warm water to obtain the individual single whiskers. Because thereacted compound of aluminum and potassium dissolves in water. That is,the slurry is subjected to a solid-liquid separation before addingthereto an acid. However, if the content of aluminum having undergonereaction with potassium is about 0.2% or less as calculated for an oxideby weight of potassium titanate whiskers, the separation and removalprocess can be omitted without causing any particular problem. If thestarting material were to contain too many impurities which formwater-soluble compounds with potassium, the masses are preferablyimmersed in cold or warm water, and the slurry is then subjected tosolid-liquid separation before adding thereto an acid to exclude thewater-soluble compounds. Potassium titanate whiskers with improvedproperties are obtained in this way.

If a starting material containing niobium and aluminum in such amountthat Al₂ O₃ /Nb₂ O₅ molar ratio is less than 0.7 was to be used, analuminum compound is added therein to control the Al₂ O₃ /Nb₂ O₅ ratioto fall in the range of from 0.7 to 1.2. There can be expected nofurther improvement in the properties of the resulting potassiumtitanate whiskers even if aluminum is added to give the Al₂ O₃ /Nb₂ O₅molar ratio is higher than 1.2. Moreover, the addition of aluminum inexcess results in too much formation of potassium aluminate which makesit difficult to separate the mass whisker into individual singlewhiskers. The aluminum compounds to be added include aluminum sulfate,aluminum chloride, aluminum nitrate, aluminum hydroxide, aluminum oxide,and aluminates.

The total amount for niobium and aluminum is not particularly limited,provided that the molar ratio thereof fall in the range defined above.However, it can be seen that the total amount for both depends on theniobium having incorporated in the starting material. In general, thestarting material contain about 0.6% niobium as calculated for Nb₂ O₅ atmaximum, and in such a case, the total content For niobium and aluminumwould be about 0.9% as calculated for an oxide.

The potassium titanate preferably contains as few impurities other thanniobium and aluminum as possible. However, potassium titanate whiskershaving an impurity content of 1% or less as calculated for an oxide poseno particular problem. If impurities are incorporated in the potassiumtitanate whiskers at a concentration exceeding 1%, mechanical strengthis reduced even if aluminum and niobium were to be incorporated at asuch amount that Al₂ O₃ /Nb₂ O₅ molar ratio is 0.7 or higher. Thereinforcing effect of such potassium titanate whiskers for a compositematerial is also reduced. Furthermore, in such a case, the strength of aheat-treated composite material comprising a light alloy and potassiumtitanate whiskers is considerably reduced and hence the reinforcingeffect by whiskers is also reduced. Such an unfavorable result isparticularly pronounced when an element other than quadrivalent ones, orwhen an element having an ionic radius differing from that of Ti⁴⁺ ionby 10% or more is dissolved in the crystal structure of potassiumtitanate whiskers, or when a quadrivalent element having weaker affinitywith oxygen than titanium is dissolved in the crystal structure ofpotassium titanate whiskers. It is believed that such elementsenumerated above accelerate the reduction of potassium titanate whiskersat the production of a composite of the whiskers with light alloys or atthe heat treatment subsequent thereto, thereby lowering the strength ofthe whiskers. As a result, a poor reinforcing effect results.

The potassium titanate whiskers to be used in the present inventionpreferably have an average length of 5 μm or more and an average aspectratio (average length/average diameter) of 10 or more.

The light alloys to be used in the present invention include aluminumbased light alloys, such as Al--Mg based alloys, Al--Mn based alloys,Al--Si based alloys, Al--Mg--Si based alloys, Al--Cu based alloys,Al--Cu--Si based alloys, and Al--Cu--Mg--Ni based alloys. That is, anyaluminum alloy used in general can be used without any particularrestriction, and magnesium alloys can also be used.

A composite of the potassium hexatitanate whiskers and a light alloy canbe produced by any of the known processes, such as a powderalloylurgical method and a squeeze casting method.

If composite materials are to be produced by a squeeze casting method, afiber preform must be formed preliminarily. If this is the case, asintered fiber preform is preferably used in place of a fiber preformthat is prepared with an inorganic binder being added, and the reason isas follows: if a fiber preform is prepared using an inorganic binder,the latter will react with the potassium titanate fibers to reduce theirstrength and reinforcing effect inherent in the fibers cannot beeffectively imparted to light alloys; Furthermore, the high reactivityof the inorganic binder with alloy components will lead to unevenpresence of the alloy components or they will interfere with theformation of interalloylic compounds that affect the intendedimprovement in the strength of the matrix, whereby the desiredeffectiveness of heat treatment cannot be easily attained.

A sintered preform can be prepared in the following manner: an organicbinder is added to a reinforcing material containing potassium titanate;after blending, the mixture is baked at a temperature of 800°-1100° C.,whereupon the potassium titanate fibers are sintered to an adequateextent, thereby yielding a melt casting preform having satisfactorycharacteristics.

The potassium hexatitanate fibers and a light alloy are used in suchproportions that the volume fraction of the potassium hexatitanatefibers is in the range of 5-40%. If the volume fraction of the potassiumhexatitanate fibers is less than 5%, their effectiveness in reinforcingthe matrix alloy is very small. If, on the other hand, the volumefraction of the potassium hexatitanate fibers is more than 40%, lesserimprovement is achieved in strength as the volume fraction of thepotassium hexatitanate fibers increases. In addition, an increasing useof the potassium hexatitanate fibers results in a higher cost ofcomposite materials.

In the process of producing the alloy-based composite material accordingto the present invention, additional reinforcing materials, such asshort alumina fibers, short crystalline alumina silica fibers, siliconcarbide whiskers, silicon nitride whiskers, aluminum borate whiskers,long alumina fibers, carbon fibers, silicon carbide fibers, and tyrannofibers may be used as well. The combined use of these additionalwhiskers with the potassium hexatitanate whiskers of the presentinvention can further provide favorable effects to the potassiumhexatitanate whisker reinforced alloy matrix composite material of thepresent invention, such as an improvement in strength and wearresistance, or a lowering of thermal expansion coefficient.

The thermoplastic resin for use in the present invention include generaluse plastics such as polypropylene, ABS, and vinyl chloride resins;engineering plastics such as polyamide, polyacetal, polyethyleneterephthalate, polybutylene terephthalate, polyphenylene sulfide,polycarbonate, and liquid crystal polymers; and alloys thereof. Thethermoplastic resins used in common can be freely used as desired.

The plastics and the potassium hexatitanate whiskers are mixed at such aratio that the potassium hexatitanate whiskers are incorporated at aweight fraction of from 3 to 50%. If the potassium hexatitanate whiskersis insufficiently added as to account for less than 3% by weight, thereinforcing effect to be imparted to the plastics becomes too little,whereas an addition thereof in excess as to exceed 50% by weight notonly impairs the moldability of the mixture but also leads to theformation of a costly composite material.

In the production of the composite material according to the presentinvention, it is preferred to use potassium titanate whiskers which istreated with a surface-treating agent such as a silane coupling agent.The potassium titanate whiskers can be incorporated into the compositematerial by either using potassium titanate whiskers having beensurface-treated in advance, or adding the surface treating agent at thekneading step of potassium titanate whiskers with plastics. It ispreferred, to use potassium titanate whiskers which have previously beensubjected to surface treatment because the effect of thesurface-treating agent can be more fully exhibited.

The silane coupling agent suitable for use in the present inventioninclude, for example, amino-type silane coupling agents such asγ-aminopropyl triethoxysilane and N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane; epoxy-type silane coupling agents such asγ-glycidoxypropyl trimethoxysilane and β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane; vinyl-type silane coupling agents such asvinyltrimethoxysilane, vinyltriethoxysilane, andvinyl-tris(2-methoxyethoxy)silane; mercapto-type silane coupling agentssuch as γ-mercaptopropyl trimethoxysilane; and acrylic silane couplingagents such as γ-methacryloxypropyl trimethoxysilane. An amount of thosesilane coupling agents is preferably 2.0% by weight or less with respectto the potassium titanate whiskers. An addition in excess does notprovide further improvement.

The composite material according to the present invention may containdepending on the purpose, one or more of various types of additives suchas fire retardants, thermal stabilizers, and lubricants, in a range thattheir addition does not impair the properties inherent in the compositematerial. Otherwise, other fillers may be added with an aim to Impartthe desired properties to the composite. Because the potassium titanatewhiskers according to the present invention exhibit considerablyimproved mechanical strength as compared with the conventional whiskers,a lower quantity can be added to plastics to obtain a comparativeeffects. Furthermore, a composite having a mechanical strength farhigher than any using conventional potassium titanate whiskers can alsobe realized.

The present invention is described in further detail below referring tosome Examples. However, it should not be construed to be limiting thescope of the invention. In the following examples, all percentages andparts are by weight unless otherwise stated.

EXAMPLE 1

A slurry of 4700 g of a metatitanic acid comprising 30% of TiO₂, 0.129%of Nb₂ O₅, and 0.017% of Al₂ O₃ was added 18 ml of an aqueous aluminumsulfate solution containing 50 g/l of aluminum as Al₂ O₃, and theresulting mixture was stirred for 20 minutes. After adding thereto 730 gof potassium carbonate powder and mixing, the slurry was spray-driedwhile maintaining the temperature at the inlet to a range of from 270°to 280° C. and that at the outlet from 80° to 85° C. The powder thusobtained was charged into an alumina crucible, and was placed in anelectric furnace for baking. The baking was conducted by heating thepowder at a rate of 200° C./hour, maintaining at 940° C. for 1 hour, andcooling at a rate of 150° C./hour.

The resulting baked product was then dipped into 12 l of hot watermaintained in a stainless steel vessel, and after 5 hours of immersion,the slurry was stirred at 4000 rpm for 1 hour using a T. K. Homomixer (ahigh-shear type mixer manufactured by Tokushu kika Kogyo Co., Ltd.). Thetemperature of the slurry was then adjusted to 60° C., and5N-hydrochloric acid was added therein dropwise to adjust the pH valueto 9.4. As further stirring increases the pH value due to the elution ofpotassium ions from between the layers of the potassium tetratitanate,hydrochloric acid was continually added dropwise every 30 minutes afterthe first addition thereof, so that the :increment in pH value might notexceed 0.1. In this manner the slurry was adjusted to the pH to 9.4. Apolymeric flocculant (trade name: Hymolock MP-173H, a product of HYMOCorporation) was added into the resulting slurry at an amount of 0.2% byweight to the potassium titanate whiskers, to thereby effectflocculation of the whiskers.

The whiskers thus obtained was filtered, washed, and baked at 1000° C.for 1 hour. The resulting product was dispersed into 10 l of hot water,and stirred for 30 minutes using T. K. Homomixer to obtain the fibersdispersed in the water. After adjusting the pH value of the dispersionto 4 by adding dropwise 1N-hydrochloric acid, whiskers were obtained byfiltration, washing, and drying. Identification of the resulting fibersby X-ray diffraction revealed that the final product were single phaseof potassium hexatitanate. SEM (Scanning Electron Microscope)observation revealed the whiskers to average length of 14 μm and anaverage diameter of 0.4 μm. The specific surface area as measured by BETmethod was 4.4 m² /g.

Chemical analysis of the potassium titanate whiskers thus obtainedyielded 0.356% Nb₂ O₅, 0.093% Al₂ O₃, 0.006% Fe₂ O₃, 0.008% MgO, 0.085%CaO, 0.012% ZnO, 0.008% P₂ O₅, and 0.006% SiO₂ as impurities, and themolar ratio of TiO₂ /K₂ O was found to be 6.0.

The value of d₇₁₂ -spacing was found to be 1.6645 Å by powder X-raydiffraction using 99.9% pure high purity silicon powder as the internalstandard.

Then 20 parts of the potassium titanate whiskers obtained above, whichwas surface-treated with 0.7% γ-aminopropyl trimethoxysilane, wascharged together with 80 parts of a Nylon 66 resin (trade name: UBENylon 2020B, a product of Ube Industries, Ltd.) into a twin-screwextruder AS-30 (manufactured by Nakatani Kikai Co., Ltd.), to obtainpellets thereof by melting and kneading at 280° C.

Comparative Example 1

A slurry of 4700 g of a metatitanic acid comprising 30% of TiO₂, 0.129%of Nb₂ O₅, and 0.017% of Al₂ O₃ was added 730 g of potassium carbonatepowder, and the resulting mixture was mixed. The slurry thus obtainedwas spray-dried while maintaining the temperature at the inlet to arange of from 270° to 280° C. and that at the outlet from 80° to 85° C.The powder thus obtained was charged into an alumina crucible, and wasplaced in an electric furnace for baking. The baking was conducted byheating at a rate of 200° C./hour, maintaining at 940° C. for 1 hour,and cooling at a rate of 150° C./hour.

The resulting baked product was then dipped into 12 l of hot watermaintained in a stainless steel vessel, and after 5 hours of immersion,the slurry was stirred at 4000 rpm for 1 hour using a T. K. Homomixer.The temperature of the slurry was then adjusted to 60° C., and5N-hydrochloric acid was added therein dropwise to adjust the pH valueto 9.4. Because a further stirring increases the pH value due to theelution of potassium ions from between the layers of the potassiumtetratitanate, hydrochloric acid was continually added dropwise every 30minutes after the first addition thereof, so that the increment in pHmight not exceed 0.1. In this manner the slurry was adjusted to the pHvalue to 9.4. A polymeric flocculant (trade name: Hymolock MP-17, aproduct of HYMO Corporation) was added into the resulting slurry at anamount of 0.2% by weight to the potassium titanate whiskers, to therebyeffect flocculation of the whiskers.

The whiskers thus obtained was filtered and baked at 1000° C. for 1hour. The resulting product was dispersed into 10 l of hot water, andstirred for 30 minutes using T. K. Homomixer to obtain the fibersdispersed in the water. After adjusting the pH value of the dispersionto 4 by adding dropwise 1N hydrochloric acid, potassium titanatewhiskers were obtained by filtration, washing, and drying. SEM (ScanningElectron Microscope) observation revealed the whiskers to an averagelength of 15 μm and an average diameter 0.4 μm. The specific surfacearea as measured by BET method was 4.5 m² /g.

Chemical analysis of the potassium titanate whiskers thus obtainedyielded 0.355% Nb₂ O₅, 0.040% Al₂ O₃, 0.006% Fe₂ O₃, 0.009% MgO, 0.083%CaO, 0.012% ZnO, 0.009% P₂ O₅, and 0.006% SiO₂ as impurities, and themolar ratio of TiO₂ /K₂ O was found to be 6.0.

The value of d₇₁₂ -spacing was found to be 1.6657 Å by powder X-raydiffraction using 99.9% pure high purity silicon powder as the internalstandard. The value 1.6657 Å is larger than that obtained for potassiumtitanate in Example 1, and it is believed that this increase ind-spacing is caused by dissolving of Al³⁺ in a crystal lattice ofpotassium titanate, i.e., because Al³⁺ ion is smaller than Ti³⁺ in iondiameter and substituted for Ti³⁺ in the crystal lattice of potassiumtitanate. Then 20 parts of the potassium titanate whiskers obtainedabove, which have been surface-treated with 0.7% γ-aminopropyltrimethoxysilane, was charged together with 80 parts oF a Nylon 66 resin(trade name: UBE Nylon 2020B, a product of Ube Industries, Ltd.) into atwin-screw extruder AS-30 (manufactured by Nakatani Kikai Co., Ltd.), toobtain pellets thereof by melting and kneading at 280° C.

The pellets thus obtained in Example 1 and Comparative Example 1 weredried in a vacuum drier at 120° C. for 12 hours, and were then injectionmolded using an injection molding machine Model SAV-30-30 (manufacturedby Yamashiro Seiki Seisakusho) at a cylinder temperature of from 260° to280° C. and a mold surface temperature of 80° C. Thus were obtained testspecimens for use in testing tensile strength and flexural strength. InTable 1 are given the strength measured on dried test pieces.

                  TABLE 1                                                         ______________________________________                                                       Example 1                                                                             Comp. Example 1                                        ______________________________________                                        Tensile Strength (kgf/cm2)                                                                     1210      1020                                               Flexural Strength (kgf/cm2)                                                                    2190      1750                                               Flexural modulus (kgf/cm2)                                                                     94000     80500                                              ______________________________________                                    

Potassium titanate whiskers were extracted from the test pieces tomeasure the lengths. The potassium titanate whiskers from the test pieceof Example 1 were found to have an average length of 7.3 μm, and thosefrom the one of Comparative Example 1 were found to be 7.5 μm onaverage; hence no significant difference was observed in the length ofthe whiskers.

EXAMPLE 2

Potassium titanate whiskers were synthesized following the process asdescribed in Example 1, except that the baking temperature afterpartially removing potassium from between layers of the potassiumtetratitanate whiskers was changed from 1000° C. to 950° C. The purityand the chemical composition of the resulting potassium titanatewhiskers were the same as those of the whiskers obtained in Example 1.

Then 20 parts of the potassium titanate whiskers obtained above, whichwas further surface-treated with 0.6% γ-glycidoxypropyltrimethoxysilane, was charged together with 80 parts of a polyacetalresin (trade name: Duracon M90, a product of Polyplastics Co., Ltd.)into a twin-screw extruder AS-30 (manufactured by Nakatani Kikai Co.,Ltd.), to obtain pellets thereof by melting and kneading at 220° C.

Comparative Example 2

Potassium hexatitanate whiskers having a tunnelstructure weresynthesized under conditions similar to those used in the process ofComparative Example 1, except for using titanium oxide in anatase form(containing 0.157% Nb₂ O₅ and 0.032% Al₂ O₃) as the titanium source. SEMobservation of the whiskers thus obtained revealed that they have anaverage length of 15 μm and an average diameter of 0.4 μm. Chemicalanalysis of the potassium titanate whiskers yielded 0.131% Nb₂ O₅,0.026% Al₂ O₃, 0.012% Fe₂ O₃, 0.014% MgO, 0.094% CaO, 0.014% ZnO, 0.010%P₂ O₅, and 0.013% SiO₂ as impurities, and the molar ratio of TiO₂ /K₂ Owas found to be 6.0.

Then 20 parts of the potassium titanate whiskers obtained above, whichwas further surface-treated with 0.6% γ-glycidoxypropyltrimethoxysilane, was charged together with 80 parts of a polyacetalresin (trade name: Duracon M90, a product of Polyplastics Co., Ltd.)into a twin-screw extruder AS-30 (manufactured by Nakatani Kikai Co.,Ltd.), to obtain pellets thereof by melting and kneading at 220° C.

The pellets thus obtained in Example 2 and Comparative Example 2 weredried in a vacuum drier at 110° C. for 10 hours, and were then injectionmolded using an injection molding machine Model SAV-30-30 (manufacturedby Yamashiro Seiki Seisakusho) at a cylinder temperature of 210° C. anda mold surface temperature of 80° C. Thus were obtained test specimensfor use in tests for tensile strength and flexural strength. In Table 2are given the results obtained in the strength tests. Potassium titanatewhiskers were extracted from the test pieces to measure their lengths,however, no significant difference was observed between the whiskers ofExample 2 and of Comparative Example 2.

                  TABLE 2                                                         ______________________________________                                                       Example 2                                                                             Comp. Example 2                                        ______________________________________                                        Tensile Strength (kgf/cm2)                                                                      1210      960                                               Flexural Strength (kgf/cm2)                                                                     2020     1630                                               Flexural modulus (kgf/cm2)                                                                     102000    78500                                              ______________________________________                                    

EXAMPLE 3

Potassium hexatitanate whiskers having a tunnelstructure weresynthesized under conditions similar to those of the process in Example1, except for using titanium oxide in anatase form (containing 0.157%Nb₂ O₅ and 0.072% Al₂ O₃) as the titanium source. SEM observation of thewhiskers thus obtained revealed that they have an average length of 15μm and an average diameter of 0.4 μm. Chemical analysis of the potassiumtitanate whiskers yielded 0.130% Nb₂ O₅, 0.055% Al₂ O₃, 0.008% Fe₂ O₃,0.009% MgO, 0.101% CaO, 0.010% ZnO, 0.010% P₂ O₅, and 0.007% SiO₂ asimpurities, and the molar ratio of TiO₂ /K₂ O was found to be 6.0.

Then 15 parts of the potassium titanate whiskers obtained above, whichwas further surface-treated with 0.6% γ-glycidoxypropyltrimethoxysilane, was charged together with 85 parts of a polycarbonateresin (trade name: Iupilon S-2000, a product of Mitsubishi Gas ChemicalCompany Inc.) into a twin-screw extruder AS-30 (manufactured by NakataniKikai Co., Ltd.), to obtain pellets thereof by melting and kneading at280° C.

Comparative Example 3

15 parts of the potassium titanate whiskers used in Comparative Example2 (a product having surface-treated with 0.6% γ-glycidoxypropyltrimethoxysilane) was charged together with 85 parts of a polycarbonateresin (trade name: Iupilon S-2000, a product of Mitsubishi Gas ChemicalCompany Inc.) into a twin-screw extruder AS-30 (manufactured by NakataniKikai Co., Ltd,), to obtain pellets thereof by melting and kneading at280° C.

The pellets obtained in Example 3 and Comparative Example 3 were theninjection molded using an injection molding machine Model SAV-30-30(manufactured by Yamashiro Seiki Seisakusho) at a cylinder temperatureof 290° C. and a mold face temperature of 90° C. Thus were obtained testspecimens for use in tests for tensile strength and flexural strength.In Table 3 are given the results obtained in the strength tests.Potassium titanate whiskers were extracted from the test pieces tomeasure their lengths, however, no significant difference was observedbetween the whiskers of Example 3 and of Comparative Example 3.

                  TABLE 3                                                         ______________________________________                                                       Example 3                                                                             Comp. Example 3                                        ______________________________________                                        Tensile Strength (kgf/cm2)                                                                      920       750                                               Flexural Strength (kgf/cm2)                                                                     1630      1310                                              Flexural modulus (kgf/cm2)                                                                     82500     67000                                              ______________________________________                                    

EXAMPLE 4

20 parts of the potassium hexatitanate whiskers obtained in Example 1having surface-treated with 0.5% γ-glycidoxypropyltrimethoxysilane wascharged together with 80 parts of a poly butyleneterephthalate resin(trade name: Duranex 2000, a product of Polyplastics Co., Ltd.) into atwin-screw extruder AS-30 (manufactured by Nakatani Kikai Co., Ltd.), toobtain pellets thereof by melting and kneading at 250° C.

Comparative Example 4

20 parts of the potassium hexatitanate whiskers obtained in ComparativeExample 1 having surface-treated with 0.5%γ-glycidoxypropyltrimethoxysilane was charged together with 80 parts ofa poly butyleneterephthalate resin (trade name: Duranex 2000, a productof Polyplastics Co., Ltd.) into a twin-screw extruder AS-30(manufactured by Nakatani Kikai Co., Ltd.), to obtain pellets thereof bymelting and kneading at 250° C.

The pellets obtained in Example 4 and Comparative Example 4 were theninjection molded using an injection molding machine Model SAV-30-30(manufactured by Yamashiro Seiki Seisakusho) at a cylinder temperatureof 250° C. and a mold face temperature of 70° C. Thus were obtained testspecimens for use in tests For tensile strength and flexural strength.In Table 4 are given the results obtained in the strength tests.Potassium titanate whiskers were extracted from the test pieces tomeasure their lengths, however, no significant difference was observedbetween the whiskers of Example 4 and of Comparative Example 4.

                  TABLE 4                                                         ______________________________________                                                       Example 4                                                                             Comp. Example 4                                        ______________________________________                                        Tensile Strength (kgf/cm2)                                                                      1030      820                                               Flexural Strength (kgf/cm2)                                                                     1980      1510                                              Flexural modulus (kgf/cm2)                                                                     106500    78500                                              ______________________________________                                    

EXAMPLE 5

7 parts of the potassium hexatitanate whiskers obtained in Example 1 wascharged together with 93 parts of a polypropylene resin (trade name: UBEPolypro J709HK, a product of Ube Industries, Ltd.) into a twin-screwextruder AS-30 (manufactured by Nakatani Kikai Co., Ltd.), to obtainpellets thereof by melting and kneading at 220° C.

Comparative Example 5

Seven parts of the potassium hexatitanate whiskers obtained inComparative Example 1 was charged together with 93 parts of apolypropylene resin (trade name: UBE Polypro J709HK, a product of UbeIndustries, Ltd.) into a biaxial extruder AS-30 (manufactured byNakatani Kikai Co., Ltd.), to obtain pellets thereof by melting andkneading at 220° C.

The pellets obtained in Example 5 and Comparative Example 5 were theninjection molded using an injection molding machine Model SAV-30-30(manufactured by Yumashiro Seiki Seisakusho) at a cylinder temperatureof 220° C. and a mold face temperature of 60° C. Thus were obtained testspecimens for use in tests for tensile strength and flexural strength.In Table 5 are given the results obtained in the strength tests.Potassium titanate whiskers were extracted from the test pieces tomeasure their lengths, however, no significant difference was observedbetween the whiskers of Example 5 and of Comparative Example 5.

                  TABLE 5                                                         ______________________________________                                                       Example 5                                                                             Comp. Example 5                                        ______________________________________                                        Tensile Strength (kgf/cm2)                                                                      410       310                                               Flexural Strength (kgf/cm2)                                                                     470       390                                               Flexural modulus (kgf/cm2)                                                                     24000     17000                                              ______________________________________                                    

EXAMPLE 6

A slurry of 4700 g of a metatitanic acid containing 30% of TiO₂, 0.129%of Nb₂ O₅, and 0.017% of Al₂ O₃ was added 18 ml of an aqueous aluminumsulfate solution containing 50 g/l of aluminum as Al₂ O₃, and theresulting mixture was stirred for 20 minutes. After adding thereto 930 gof potassium carbonate powder and mixing, the slurry was spray-driedwhile maintaining the temperature at the inlet to a range of from 270°to 280° C. and that at the outlet from 80° to 85° C. The powder thusobtained was charged into an alumina crucible, and was placed in anelectric furnace for baking. The baking was conducted by heating thepowder at a rate of 150° C./hour, maintaining at 950° C. for 1 hour, andcooling at a rate of 150° C./hour.

The resulting baked product was then dipped into 10 l of hot watermaintained in a stainless steel vessel, and after 5 hours of immersion,the slurry was stirred at 600 rpm for 1 hour. The temperature of theslurry was then adjusted to 60° C., and 5N-hydrochloric acid was addedtherein dropwise to adjust the pH value to 9.5. As a further stirringincreases the pH value due to the elution of potassium ions from betweenthe layers of the potassium tetratitanate, hydrochloric acid wascontinually added dropwise every 30 minutes after the first additionthereof, so that the increment in pH value might not exceed 0.1. In thismanner the slurry was adjusted the pH value to 9.5.

The whiskers thus obtained was filtered, and baked at 950° C. for 2hours. The resulting product was dispersed into 10 l of hot water, andafter adjusting the pH value of the dispersion to 4 by adding dropwise1N-hydrochloric acid, whiskers were obtained by filtration, washing, anddrying. Identification of the resulting fibers by X-ray diffractionrevealed that the final products were single phase of potassiumhexatitanate. SEM (Scanning Electron Microscope) observation revealedthe whiskers to have an average length of 50 μm and an average diameterof 1.0 μm.

Chemical analysis of the potassium titanate whiskers thus obtainedyielded 0.354% Nb₂ O₅, 0.095% Al₂ O₃, 0.005% Fe₂ O₃, 0.010% MgO, 0.085%CaO, 0.012% ZnO, 0.009% P₂ O₅, and 0.006% SiO₂ as impurities, and themolar ratio of TiO₂ /K₂ O was found to be 6.0.

The value of d₇₁₂ -spacing was found to be 1.6644 Å by powder X-raydiffraction using 99.9% pure high purity silicon powder as the internalstandard.

The potassium titanate whiskers thus obtained was added into water togive a dispersion containing the potassium titanate whiskers at aconcentration of 150 g/l, and after adding further therein 0.1% byweight of a dispersant (trade name: Nopcosant RFA, a product of SanNopco Co., Ltd.), stirring was conducted for 30 minutes using ahomomixer to obtain a homogeneous dispersion. Then, polyvinyl alcohol(trade name: Kuraray Poval 205, a product from Kuraray Co., Ltd.) wasadded to the potassium titanate whiskers at an amount equivalent to 8.0%by weight. The slurry thus obtained was poured into a mold made of avinyl chloride resin to obtain a preform by suction filtering. Thepreform was then replaced in a mold, and pressure was applied thereto toobtain a preform containing the whiskers at a volume fraction of 30%.Then the resulting preform was released from the mold, dried, and thenheated to 950° C. to obtain a baked preform.

The thus obtained preform was pre-heated at ca. 800° C., and was thenreplaced into a mold which have been maintained at a temperature of 350°C. To the preform in the mold was poured a molten of A132 (U.S.standard) material at ca. 800° C. while applying thereto a pressure of1000 kg/cm², which was followed by rapid cooling to obtain a solidcomposite material.

EXAMPLE 7

High quality potassium hexatitanate whiskers were synthesized underconditions similar to those of the process in Example 6, except forchanging the baking temperature after partially removing potassium frombetween the layers of potassium tetratitanate whiskers from 950° C. to850° C. The purity and the chemical composition of the resultingpotassium titanate whiskers were the same as those obtained in Example6.

A preform containing whiskers at a volume fraction of 30% was preparedin a similar procedure as that employed in Example 6. Thus, the preformhaving been preheated to ca. 800° C. was placed in a mold maintained ata temperature of 350° C., and a molten A132 (U.S. standard) material atca. 800° C. was poured thereto under a pressure maintained at 1000kg/cm², which was followed by rapid cooling to obtain a compositematerial.

Comparative Example 6

A slurry of 4700 g of a metatitanic acid containing 30% of TiO₂, 0.129%of Nb₂ O₅, and 0.017% of Al₂ O₃ was added 930 g of potassium carbonatepowder and mixed. The resulting slurry was spray-dried while maintainingthe temperature at the inlet to a range of from 270° to 280° C. and thatat the outlet from 80° to 85° C. The powder thus obtained was chargedinto an alumina crucible, and was placed in an electric furnace forbaking. The baking was conducted by heating the powder at a rate of 150°C./hour, maintaining at 1050° C. for 1 hour, and cooling at a rate of150° C./hour.

The resulting baked product was then dipped into 10 l of hot watermaintained in a stainless steel vessel, and after 5 hours of immersion,the slurry was stirred at 600 rpm. The temperature of the slurry wasthen adjusted to 60° C., and 5N-hydrochloric acid was added thereindropwise to adjust the pH value to 9.5. As a further stirring increasesthe pH value due to the elution of potassium ions from between thelayers of the potassium tetratitanate, hydrochloric acid was continuallyadded dropwise every 30 minutes after the first addition thereof, sothat the increment in pH value might not exceed 0.1. In this manner theslurry was adjusted the pH value to 9.5.

The whiskers thus obtained was filtered, and baked at 950° C. for 2hours. The resulting product was dispersed into 10 l of hot water, andafter adjusting the pH value of the dispersion to 4 by adding dropwise1N-hydrochloric acid, whiskers were obtained by filtration, washing, anddrying. Identification of the resulting fibers by X-ray diffractionrevealed that the final products were single phase of potassiumhexatitanate. SEM (Scanning Electron Microscope) observation revealedthe whiskers to have an average length of 50 μm and an average diameterof 1.0 μm.

Chemical analysis of the potassium titanate whiskers thus obtainedyielded 0.354% Nb₂ O₅, 0.042% Al₂ O₃, 0.005% Fe₂ O₃, 0.010% MgO, 0.085%CaO, 0.012% ZnO, 0.009% P₂ O₅, and 0.006% SiO₂ as impurities, and themolar ratio of TiO₂ /K₂ O was found to be 6.0.

The value of d₇₁₂ -spacing was found to be 1.6658 Å by powder X-raydiffraction using 99.9% pure high purity silicon powder as the internalstandard. The value 1.6658 Å is larger than that obtained for potassiumtitanate in Example 1, and it is believed this increase in d-spacing iscaused by dissolving of Al³⁺ in a crystal lattice of potassium titanate,i.e., because Al³⁺ ion smaller than Ti³⁺ in ion diameter and substitutedfor Ti³⁺ in the crystal lattice of potassium titanate.

A preform containing whiskers at a volume fraction of 30% was preparedin a similar procedure as that employed in Example 6. Thus, the preformhaving been preheated to ca. 800° C. was replaced in a mold maintainedat a temperature of 350° C., and a molten A132 (U.S. standard) materialat ca. 800° C. was poured thereto under a pressure maintained at 1000kg/cm², which was followed by rapid cooling to obtain a compositematerial.

EXAMPLE 8

Potassium hexatitanate whiskers having a tunnel structure weresynthesized under conditions similar to those of the process in Example6, except for using a titanium oxide in antase form (containing 0.157%Nb₂ O₅ and 0.072% Al₂ O₃) as the titanium source. SEM observation of thewhiskers thus obtained revealed that they have an average length of 40μm and an average diameter of 1.0 μm. Chemical analysis of the potassiumtitanate whiskers yielded 0.130% Nb₂ O₅, 0.055% Al₂ O₃, 0.008% Fe₂ O₃,0.009% MgO, 0.101% CaO, 0.010% ZnO, 0.010% P₂ O₅, and 0.007% SiO₂ asimpurities, and the molar ratio of TiO₂ /K₂ O was found to be 6.0.

A preform for sintering containing whiskers at a volume fraction of 30%was prepared in a similar procedure as that employed in Example 6. Thus,the preform having been preheated to ca. 800° C. was replaced in a moldmaintained at a temperature of 350° C., and a molten A132 (U.S.standard) material at ca. 800° C. was poured thereto under a pressuremaintained at 1000 kg/cm², which was immediately followed by a rapidcooling to obtain a composite material.

Comparative Example 7

Potassium hexatitanate whiskers having a tunnel structure weresynthesized under conditions similar to those of the process inComparative Example 6, except for using titanium oxide in anatase form(containing 0.157% Nb₂ O₅ and 0.032% Al₂ O₃) as the titanium source. SEMobservation of the whiskers thus obtained revealed that they have anaverage length of 40 μm and an average diameter of 1.0 μm. Chemicalanalysis of the potassium titanate whiskers yielded 0.131% Nb₂ O₅,0.026% Al₂ O₃, 0.012% Fe₂ O₃, 0.014% MgO, 0.094% CaO, 0.014% ZnO, 0.010%P₂ O₅, and 0.013% SiO₂ as impurities, and the molar ratio of TiO₂ /K₂ Owas found to be 6.0.

A preform containing whiskers at a volume fraction of 30% was preparedin a similar procedure as that employed in Example 6. Thus, the preformhaving been preheated to ca. 800 ° C. was replaced in a mold maintainedat a temperature of 350° C., and a molten A132 (U.S. standard) materialat ca. 800 ° C. was poured thereto under a pressure maintained at 1000kg/cm², which was followed immediately thereafter by a rapid cooling toobtain a composite material.

Reference Example 1

A preform was prepared under conditions similar to those used in Example6, except for using TISMO N (potassium titanate whiskers manufactured byOtsuka Chemical Co., Ltd. ) as potassium titanate whiskers. Chemicalanalysis of this commercially available potassium titanate whiskersyielded 0.120% Nb₂ O₅, 0.062% Al₂ O₃, 0.010% Fe₂ O₃, 0.027% MgO, 0.111%CaO, 0.001% ZnO, 0.089% P₂ O₅, and 0.005% SiO₂ as impurities, and themolar ratio of TiO₂ /K₂ O was found to be 6.3. Furthermore, this whiskerwas identified by X-ray diffraction analysis to be a two-phase mixtureconsisting of potassium hexatitanate whiskers and titanium dioxide inanatase form.

Then, the preform having been preheated to ca. 800° C. was replaced in amold maintained at a temperature of 350° C., and a molten A132 (U.S.standard) material at ca. 800° C. was poured thereto under a pressuremaintained at 1000 kg/cm², which process was followed immediatelythereafter by rapid cooling to obtain a composite material.

Reference Example 2

A preform was prepared under conditions similar to those used in Example6, except For using Tofica Y (potassium titanate whiskers manufacturedby Nippon Whisker Co., Ltd.) as potassium titanate whiskers. Chemicalanalysis of this commercially available potassium titanate whiskersyielded 0.222% Nb₂ O₅, 0.276% Al₂ O₃, 1.487% Fe₂ O₃, 0.008% NiO, 0.034%MnO, 0.073% MgO, 0.053% CaO, 0.011% ZnO, 0.131% P₂ O₅, and 0.010% SiO₂as impurities, and the molar ratio of TiO₂ /K₂ O was found to be 6.2.Furthermore, this whisker was identified by X-ray diffraction analysisto be a two-phase mixture consisting of potassium hexatitanate whiskersand titanium dioxide in anatase form.

Then, the preform having been preheated to ca. 800° C. was replaced in amold maintained at a temperature of 350° C., and a molten A132 (U.S.standard) material was poured thereto at ca. 800° C. under a pressuremaintained at 1000 kg/cm², which process was followed immediatelythereafter by rapid cooling to obtain a composite material.

The composite materials obtained in Examples 6, 7, and 8, andComparative Examples 6 and 7, as well as in Reference Examples 1 and 2were machined to obtain test pieces for a tensile test.

The tensile strength at room temperature of the test pieces before heattreatment and after T71 heat treatment (comprising steps of solutionheat treatment at 525° C. for 16 hours, water-quenching, and artifialaging treatment at 188° C. for 5 hours). The test results are given inTable 6.

                  TABLE 6                                                         ______________________________________                                                     Tensile Strength (MPa)                                                        Before Heat                                                                            After Heat                                                           Treatment                                                                              Treatment                                               ______________________________________                                        Example 6      381        415                                                 Example 7      370        398                                                 Example 8      388        412                                                 Comp. Ex. 6    379        330                                                 Comp. Ex. 7    382        361                                                 Ref. Ex. 1     334        270                                                 Ref. Ex. 2     325        251                                                 ______________________________________                                    

It can be seen from Table 6 that the strength of the composite materialsreinforced with the potassium titanate whiskers, according to thepresent invention are all improved in strength after heat treatment,whereas that of the Comparative Examples 6 and 7 and of the ReferenceExamples 1 and 2 are considerably reduced in strength after the heattreatment. Particularly in the case of Reference Examples 1 and 2, wherethe commercially available potassium titanate whiskers containingimpurities aluminum and niobium at a such amount that the Al₂ O₃ /Nb₂ O₅molar ratio is 0.6 or higher, the strength decreased significantly. Thisis believed due to the fact that both potassium titanate whiskers arenot single phase of potassium hexatitanate. Furthermore, in thepotassium titanate whiskers of the Reference Example 2 in particular,the presence of a large amount of impurities is found as another reasonfor the poor strength after the heat treatment.

EXAMPLE 9

A preform containing whiskers at a volume fraction of 16% was preparedin the same procedure as that employed in Example 6, using potassiumhexatitanate whiskers obtained in Example 6.

Thus, the preform having been preheated to ca. 800° C. was replaced in amold maintained at a temperature of 350° C., and a molten AC8A (JISstandard) material at ca. 800° C. was poured thereto under a pressuremaintained at 1000 kg/cm², which was followed immediately thereafter bya rapid cooling to obtain a composite material.

Comparative Example 8

A preform containing whiskers at a volume fraction of 16% was preparedin the same procedure as that employed in Example 6, using potassiumtitanate whiskers obtained in Comparative Example 6.

Thus, the preform having been heated to ca. 800° C. was placed in a moldmaintained at a temperature of 350° C., and a melt of JIS standard AC 8Amaterial at ca. 800° C. was poured thereto under a pressure maintainedat 1000 kg/cm², which was followed immediately thereafter by a rapidcooling to obtain a composite material.

The composite materials obtained in Example 9 and Comparative Example 8were machined to obtain test pieces as shown in FIG. 1 for a tensiletest.

The tensile strength at room temperature of the test pieces before heattreatment and after T6 heat treatment (comprising steps of solution heattreatment at 510° C. for 4 hours, water-quenching, and artifical agingtreatment at 170° C. for 10 hours). The test results are given in Table7.

                  TABLE 7                                                         ______________________________________                                                     Tensile Strength (MPa)                                                        Before Heat                                                                            After Heat                                                           Treatment                                                                              Treatment                                               ______________________________________                                        Example 9      372        394                                                 Comp. Ex. 8    374        350                                                 ______________________________________                                    

EXAMPLE 10

A preform containing whiskers at a volume fraction of 25% was preparedin the same procedure as that employed in Example 6, using the potassiumtitanate whiskers obtained in Example 6.

Thus, the preform having been preheated to ca. 800° C. was replaced in amold maintained at a temperature of 350° C., and a molten AC4C (JISstandard) material at ca. 770° C. was poured thereto, which was followedimmediately thereafter by a rapid cooling under a pressure maintained at1000 kg/cm² to obtain a composite material.

Comparative Example 9

A preform containing whiskers at a volume fraction of 25% was preparedin the same procedure as that employed in Example 6, using the potassiumtitanate whiskers obtained in Comparative Example 6.

Thus, the preform having been preheated to ca. 800° C. was replaced in amold maintained at a temperature of 350° C., and a molten Ac4C (JISstandard) material at ca. 770° C. was poured thereto, which was followedimmediately thereafter by a rapid cooling under a pressure maintained at1000 kg/cm² to obtain a composite material.

The composite materials obtained in Example 10 and Comparative Example 9were machined to obtain test pieces as shown in FIG. 1 for the tensiletest.

The tensile strength at room temperature of the test pieces before heattreatment and after T61 heat treatment (comprising steps of solutionheat treatment at 525° C. for 8 hours, water-quenching, and artificalaging treatment at 170° C. for 6 hours). The test results are given inTable 8.

                  TABLE 8                                                         ______________________________________                                                     Tensile Strength (MPa)                                                        Before Heat                                                                            After Heat                                                           Treatment                                                                              Treatment                                               ______________________________________                                        Example 10     362        381                                                 Comp. Ex. 9    354        327                                                 ______________________________________                                    

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope of the invention.

What is claimed is:
 1. A potassium hexatitanate whisker having a tunnelstructure containing aluminum and niobium impurities in such amountsthat the Al₂ O₃ /Nb₂ O₅ molar ratio is at least 0.6 and containing 1% orless by weight in total of impurities as oxides other than aluminum andniobium.
 2. An alloy-base composite material comprising a light alloymatrix and potassium hexatitanate whiskers having a tunnel structure,said whiskers containing aluminum and niobium impurities in such amountsthat the Al₂ O₃ /Nb₂ O₅ molar ratio is at least 0.6 and containing 1% orless by weight in total of the whiskers impurities as oxides other thanaluminum and niobium.